Embodiments of the present invention relate to devices that use light for the purpose of analysis. These devices and apparatus are used as detectors. It is convenient to refer to light as comprised of photons and this application will use the terms light and photons interchangeably. In such devices light is introduced into a vessel containing a sample; light leaving the vessel is measured. The absence or diminution of light of a particular wavelength, present upon entry into the vessel suggests absorption of light by the sample or a constituent within the sample. The presence of light at an intensity or at a wavelength not present upon entry into the vessel suggests a shift in wavelength. These changes in the light entering and leaving a vessel are characteristic of the compounds in the sample.
The term “sample” is used in the broadest sense to indicate something that one wishes to evaluate. Samples can originate with industrial materials, chemical synthesis, or may have originated from biological sources.
Vessels which receive a flow of sample over time and subject such sample flow to light for analysis purposes are called optical flow cells. These vessels can comprise a length of conduit of similar dimensions or may represent a broadening or narrowing of the conduit. Such vessels typically will have inlets and outlets for sample and inlets and outlets for light.
Chromatography is the science of separations based on differences in affinity that different compositions have to a stationary phase. High performance liquid chromatography (HPLC) is performed in columns or cartridges. Solutions in which samples are dissolved are pumped through the columns or cartridges. The columns and cartridges conduits have an inert stationary phase. The components of the sample separate as they move through the stationary phase. It desirable to detect the separated components with a detector.
This application will use the term HPLC as referring to separations at pressures up to approximately 3,000 pounds per square inch (psi). At higher pressures, it is possible to perform sharper better defined separations with greater speed. However, higher pressures, referring now to the ultra high pressure range, approximately 4,000 psi to 15,000 psi, place extreme demands on equipment.
Mechanical stresses brought on by high sample pressures and the need for sample-wetted materials that resist the wide variety of HPLC solvents and samples can result in optical-fiber based flow cell designs that are difficult to manufacture. The end of an optical fiber placing light in or taking light out may need to be subjected to the ultra high pressure. The optical fiber, in these situations, is difficult to secure. The optical fiber must also be shielded from extraneous light. The flow cell vessel must also create a flow of sample in which all the sample is exposed to an equal amount of light; that is, there are no dead volumes. These difficulties are compounded by the small scale of the devices. It is desirable to have optical sensors which have small volumes.